Naturally occurring materials tend to have defects in them, some of the most common types being vacancies and substitutes. In diamond crystals, a nitrogen vacancy center forms when a nitrogen atom substitutes for a missing carbon. The spin states of these defect centers exhibit long coherence times, as much as 1 s. These defect centers can also be introduced artificially, such as via irradiation. There are two charge states of nitrogen vacancy defects, a neutral nitrogen vacancy (NV°) and the negatively charged nitrogen vacancy (NV).